1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to a mobile communication system capable of increasing communication efficiency with an adaptive modulation and coding (AMC) scheme, and a method therefor.
2. Description of the Related Art
In order to transmit a large quantity of data to a wireless channel environment, information regarding a current channel obtained from a receiver must be sent to a transmitter at a precise time. In this arrangement, it is possible to change a modulation mode and a coding mode according to the condition of a current channel, and transmit a large amount of data without a loss in data. Here, much attention is directed to how to transmit information regarding a channel from the receiver to the transmitter with the least amount of feedback bits and without an error or delay in transmission, thereby saving wireless channel resources. Additionally, the transmitter is required to select the optimum level out of a plurality of levels of a modulation and coding scheme (MCS), using the channel information transmitted from a receiver, and to transmit a large amount of data to a wireless channel environment.
In general, a common pilot channel (CPICH) method or a cyclic redundancy check (CRC) method is adopted to identify the channel environment of a receiver and to change a combination level of an MCS level according to the detected channel environment.
FIG. 1 is a schematic block diagram of a conventional mobile communication system adopting the CPICH method. This mobile communication system includes a transmitter 10, a data channel 20, a receiver 30, and a feedback channel 22. The transmitter 10 includes an adaptive modulation and coding (AMC) unit 12. The receiver 30 includes a demodulation & decoding unit 32, a channel quality measurement unit 34, and an MCS level selection controller 36.
FIG. 2 is a view explaining a process of exchanging data between the transmitter 10 and the receiver 30.
Referring to FIGS. 1 and 2, in step 40 of FIG. 2, the transmitter 10 sends channel information together with a pilot signal CPICH, which is used to measure the quality of a channel, to the receiver 30 via a pilot channel (not shown). Then, the channel quality measurement unit 34 of the receiver 30 receives the pilot signal CPICH together with the channel information from the transmitter 10, measures a signal-to-interference ratio (SIR) or a signal-to-noise ratio (SNR) as the quality of a channel using the pilot signal CPICH, and transmits the result to the MCS level selection controller 36.
The MCS level selection controller 36 selects a level of MCS according to the quality of the channel measured by the channel quality measurement unit 34. FIG. 3 is a view of MCS levels MCS1 through MCS7, and boundary values TH1 through TH6 that define the boundaries between the MCS levels MCS1 through MCS7 in the case of W-CDMA (code division multiple access). Next, in step 42, the MCS level selection controller 36 compares each boundary value with the quality value of a channel measured by the channel quality measurement unit 34, selects a current MCS level based on the comparing result, and transmits information on the selected MCS level to the AMC unit 12 of the transmitter 10.
Referring to FIG. 1, in step 44 of FIG. 2, the AMC unit 12 sends a signal x[i], which is a transmission signal s[i] modulated and coded according to the MCS level transmitted from the MCS level selection controller 36, to the receiver 30 via the data channel 20. While passing through the data channel 20, the signal x[i] is combined with channel noise G[i] and white noise n[i] and becomes a signal y[i]. Finally, the signal y[i] is transmitted to the receiver 30.
Although not shown in FIG. 1, in step 46 of FIG. 2, either the receiver 30 sends an acknowledgement (ACK) signal to the transmitter 10 when it completely receives data from the transmitter 10 or the receiver 30 sends a signal NACK when it fails to completely receive the data. In step 48, when the transmitter 10 receives the NACK signal from the receiver 30, it resends the data to the receiver 30.
As previously mentioned, according to the CPICH method, the receiver 30 measures the quality of a current channel, using a pilot signal transmitted from the transmitter, and selects and transmits the optimum MCS level to the transmitter. Then, the transmitter determines the degree of modulation and coding according to the optimum MCS level, and returns the result to the receiver.
This conventional CPICH method, however, wastes uplink wireless channel resources because MCS selection bits corresponding to the number of MCS levels must be periodically transmitted to a transmitter at predetermined instants of time. For instance, in current W-CDMA that classifies the MCS into seven levels, the channel quality measurement unit 34 transmits three bit MCS selection bits to the transmitter at every three slots. In addition, six comparators are required to compare a measured quality value of a channel with each of the boundary values TH1 through TH6, thereby complicating the circuit structure of a mobile communication system. These comparators are installed in a receiver, i.e., a handset, which causes an increase in the power consumption of the handset.
Further, when the receiver has an error in measuring a channel and a delay in feedback, the actual conditions of the channel may be different from the measured conditions of the channel at the time when a transmitter modulates and codes a signal and transmits the result to the receiver based on channel information measured by the receiver. This will lower throughput. Here, the delay in feedback is the delay of time that is spent while the channel quality measurement unit 34 measures the quality of a channel and the MCS selection controller 34 selects the optimum MCS level according to the measured quality of the channel. In other words, the greater the number of MCS levels, the longer the time lost in selecting the optimum MCS level and the longer a delay in feedback become.
The CRC method has been recently suggested as an alternative of the CPICH. FIG. 4 is a view explaining a process of exchanging data between a receiver and a transmitter according to a conventional CRC method.
Referring to FIG. 4, in step 50, the transmitter 10 directly sends CRC information, which is used to detect an error in an actual channel, along with data to the receiver 30 without receiving any feedback regarding an MCS level according to the quality of the channel from the receiver 30. At this time, the transmitter 10 transmits the CRC information and data to the receiver 30 at the lowest MCS level. Then, in step 52, the receiver 30 checks whether the CRC information contains an error, and sends an ACK signal if no error is detected. In step 54, if the ACK signal is continuously transmitted from the receiver 30 to the transmitter 10 a predetermined number of times, the transmitter 10 raises the MCS level by one level and then, sends CRC information and data to the receiver 10. However, if the receiver 30 detects an error during the check of the CRC information, in step 56, the receiver 30 sends an NACK signal to the transmitter 10. Subsequently, in step 58, the transmitter 10 lowers the MCS level by one level and resends the data to the receiver 30.
In conclusion, according to the conventional CRC method, the transmitter 10 begins sending data together with CRC information at the lowest MCS level without information on the quality of a channel, and optimizes the MCS level in response to the ACK signal, which is transmitted from the receiver 30 as confirmation of safe receipt of the data, or the NACK signal which is transmitted from the receiver 30 when an error is detected in the CRC information. Therefore, if conditions of a channel are sufficient to transmit data at the highest MCS level, the CRC method requires a long time to achieve a desired MCS level. Also, while the level of the channel increases to a desired MCS level, the transmitter 10 transmits only a small amount of data, which does not match the transmission capability of the channel, thereby reducing data throughput.
As a result, in a conventional mobile communication system, uplink frequency resources are wasted due to the amount of feedback information regarding the quality of a channel and frequent reports on channel information. Further, it is difficult for a transmitter to use to correct channel information at a desired time due to a feedback error and a delay in the transmission of feedback information. Otherwise, an MCS level cannot be adjusted to correspond to the quality of a current channel until information is exchanged between a transmitter and a receiver for considerable time. During this time, a small amount of the transmission of data, which does not match the quality of a channel, is transmitted from a transmitter to a receiver, thereby reducing data throughput.